The invention relates to a method and apparatus for measuring the spacing between a pair of surfaces, at least one of which is formed of ferromagnetic material. The invention contemplates utilization of magnetic fields for making distance determinations, and is particularly adapatable for situations where one surface wears with respect to another during use.
The primary applicability of the invention is to machinery in which it is important that a particular clearance be maintained between two metallic surfaces that are moving relative to each other. For instance, in conical plug valves or feeders which are used in the cellulose pulp industry (for example, in connection with rotating pocket feeders for high and/or low pressure system in the continuous digestion of wood chips to cellulosic pulp, such as those sold by Kamyr A.B. of Karlstad, Sweden, or Kamyr, Inc. of Glens Falls, New York), a pocketed rotating plug is generally conical in shape and fits into a corresponding conical-shaped opening in the stationary housing. Such feeders are exposed to pressures in the range of about 1-15 bars, and it is necessary that the clearance between the rotor and the housing be kept at a minimum in order to avoid excessive leakage of cooking liquor, while at the same time being large enough so that metallic contact between the relatively moving rotor and housing does not occur. By practicing the present invention and by utilizing a control system, the rotor plug of such a feeder can be automatically adjusted in the axial direction in order to obtain the desired results, and significant monetary and technical advantages.
The invention is also applicable to other industries, for instance, where there is a need for measuring the clearance in plain bearings, and to other aspects of the pulp industry, as when measuring the gap in various grinding machines, and the like. For instance, in the pulp and paper industry, refiners (also known as defibrators) are used to grind raw cellulosic material, such as chips, into pulp, normally utilizing a stationary grinding surface and a rotating grinding surface, which surfaces may be either of conical or flat configuration. The raw material to be ground passes between the opposing surfaces. In such refiners, it is necessary that the gap between the grinding surfaces be kept at a desired distance (which can be as little as a fraction of a millimeter), and it is also necessary that this clearance be accurately measured and adjusted during operation, taking into account any wear of the grinding surfaces. The surfaces are, in most cases, rough, having grinding portions disposed in a pattern, and of course the spacing between the surfaces must be controlled so that the grinding portions do not touch each other. It is desirable to provide for automatic control of the grinding surfaces so that a desired optimum gap is kept between them.
There have been a number of previous proposals for utilizing magnetic reluctance between a transducer and a closely located ferromagnetic object for distance measurement, such as shown in U.S. Pat. No. 4,387,339. In such devices, when the magnetic field is produced by direct current, the accuracy of the measurement procedure is often disturbed by temperature variations (since they influence the permeability of the ferromagnetic material). Other prior art procedures are based on the utilization of a transducer which generates an alternating current magnetic field which induces eddy currents in an electrically conducting object. The eddy current varies with the distance to the transducer. These measurement techniques are also not necessarily precise since they can be disturbed by variations in the resistivity of the object, and the medium between the object and the transducer.
Additional problems arise in using transducers in surroundings which are deleterious to their proper functioning. For example, magnetic windings, to function properly, must not be exposed to aggressive surroundings in the measuring gap, for example, to cellulose pulp disposed in the gap between opposed relatively rotating grinding surfaces. Exposure of the windings to water, chemicals, elevated temperatures and pressure pulsations would adversely affect the proper operation of the transducer. Additional practical constraints are imposed on the disposition of transducers in such environment. For example, a material surrounding the tranducer and protecting it must not short-circuit the magnetic field; and must be formed of materials which are mechanically strong and chemically resistant at the working temperatures and pressures of the device.
Moreover, problems arise in the use of cores desirably formed of high permeable magnetic material in conjunction with surfaces thereof necessarily exposed to the surrounding high pressure and temperature, chemically abrasive environment. While it is desirable to have transducer cores of high permeability materials, it is also necessary that such materials, when exposed to the environment, do not abrade or wear differently than the grinding surface. If this occurs, potentially false measurements of the gap due to wear may arise.
According to the present invention, apparatus and methods for measuring the distance between opposing relatively moving surfaces are provided which minimize or eliminate the above-identified drawbacks and disadvantages associated with prior measuring devices. In practicing the present invention, there is provided first and second surfaces with at least the second surface being formed of magnetic material, i.e., a ferromagnetic material. A single transducer is carried by the first surface and has a core surrounded by a first coil. The poles of the core terminate substantially flush with the first surface and are spaced from the second surface to define a gap therewith. Electrical currents supplied to the first coil establish a magnetic field through the core, poles, gap and the second surface, the strength of the field being a function of the distance between the surfaces. A second coil carried by the core produces an electrical signal proportional to the strength of the field. In this manner, the gap between the first and second surfaces can be continuously monitored.
The present invention minimizes or eliminates problems associated with the potential for short-circuiting the magnetic field established by the first coil, at the same time affording an environment for the transducer which is sufficiently strong and chemically resistant to negate the adverse effects of the working environment, i.e., high pressures and working temperatures, for example, on the order of 150.degree.-180.degree. C. in a refiner for grinding cellulose pulp. To accomplish this, the transducer is disposed in and encapsulated by a stainless steel housing, in turn, disposed in a recess formed in the first member carrying the first surface. The housing is formed of stainless steel to provide low electrical conductivity. Because the magnetic field is an alternating field, the stainless steel housing confines the magnetic field inside the housing except to the extent that the field extends through the poles, across the gap and into the magnetic material of the second member carrying the second surface.
In another aspect of the present invention and in order to ensure that the transducer accurately measures the gap distance between the relatively rotating surfaces as they are worn, the present invention provides poles which project from the stainless steel housing to terminate flush with the first surface. In accordance with the present invention, the core is formed in segments wherein the pole pieces are formed of the same material as the material forming the first surface or other magnetic material having a corresponding wear resistance. Thus, while the poles may be formed of a material highly resistant to wear similarly as the first surface, and as such, they are formed of a material of lower magnetic permeability, the poles are connected to the core over a very short distance. In this manner, the magnetic field passes through a material of relatively low permeability for only a very short distance. Thus, the magentic field does not deteriorate significantly in any manner which would deleteriously affect the measurement. As a result of this construction, the poles are worn at the same rate as the surrounding material of the first surface, thereby enabling the continuous and accurate monitoring of the gap.
In a further form of the invention, the second coil produces an electrical signal which is proportional to the strength of the field and, hence, monitors the gap distance. The gap distance can then be adjusted by mechanical means in accordance with the electrical signal. In a still further form of the present invention, an amplifier regulates the current supplied to the first coil such that an alternating current voltage with constant amplitude is obtained in the second coil whereby the value of the current supplied to the first coil is proportional to the strength of the field and, hence, the gap distance. Monitoring of the electric current supplied to the first coil thus provides a signal for driving mechanical means to adjust the gap as desired.
It is therefore a primary object of the present invention to provide novel and improved methods and apparatus for determining the gap distance between opposing surfaces using electromagnetic techniques. This and further objects and advantages of the present invention will become more apparent upon reference to the following specification, claims and drawings.